The Becquerel (Bq) is an SI unit of radioactivity: one becquerel is equivalent to one radioactive decay per second. That absolutely does not make it equivalent to one hertz — the random nature of radioactive decay means you’ll never get one pulse every second — but it does make it interesting. [mihai.cuciuc] certainly thought so, when he endeavored to create a clock that would tick at one becquerel.
The result is an interesting version of a Vetinari Clock, first conceived of by [Terry Pratchett] in his Discworld books. In the books, the irregular tick of the clock is used by Lord Vetinari as a form of psychological torture. For some reason, imposing this torture on ourselves has long been popular amongst hackers.
Without an impractical amount of shielding, any one-becquerel source would be swamped by background radiation, so [mihai] had to get creative. Luckily, he is the creator of the Pomelo gamma-ray spectroscope, which allowed him to be discriminating. He’s using an Am-241 source, but just looking for the characteristic 59.5 KeV gamma rays was not going to cut it at such a low count rate. Instead he’s using two of the Pomelo solid-state scintillation as a coincidence detector, with one tuned for the Am-241’s alpha emissions. When both detectors go off simultaneously, that counts as an event and triggers the clock to tick.
How he got exactly one becquerel of activity is a clever hack, too. The Am-241 source he has is far more active than one decay per second, but by varying the distance from the gamma detector he was able to cut down to one detection per second using the inverse square law and the shielding provided by Earth’s atmosphere. The result is a time signal that is a stable one hertz… if averaged over a long enough period. For now, anyway. As the Am-241 decays away, its activity decreases, and [mihai] admits the clock loses about 0.4 seconds per day.
While we won’t be giving the prize for accuracy in this contest, we are sure Lord Vetinari would be proud. The Geiger-counter sound effect you can hear in the demo video embedded below is great touch. It absolutely increases the psychic damage this cursed object inflicts.
Any cheap chinese dupes in chat?
Bravo! [hand clap]
This is illegal in most of civilized world where radioactive materials are banned. (Wink wink ruzzians with their polonium-210 tea.)
am-241 is the stuff in smoke detectors btw
Not a problem in the united states, where you can own multiple pounds (not kilograms) or something of uranium ore.
Are granite countertops illegal in your country?
Nothing about this project is illegal in the US.
I don’t know where the civilized world ends, but I strongly suspect that taking a (one, as in a single) smoke detector apart is not illegal either. Even if it is, I don’t think anyone is ever going to bother with this.
Yeah, you can probably cite some obscure regulation in an authoritarian country like the UK, but find me an actual court case based on one smoke detector.
Radioactive ore samples are available in many places in the US, such as United Nuclear and eBay.
You can purchase a card full of radioactive glow-in-the-dark watch hands on eBay. Mine came with burn marks from all the radiation.
Well, there was that one australian case from earlier this year, which almost went to trial. The plutonium in question was from a russian smoke detector.
https://www.theguardian.com/australia-news/2025/mar/21/emmanuel-lidden-sydney-science-nerd-importing-plutonium-ntwnfb
On BBT and Young Sheldon there was the mention of the Feds showing up when Sheldon was using the radioactive part of a bunch of smoke detectors to build a nuclear reactor.
Which is a reference, of course, to the young man who did that in real life and had his project seized by the government. (And who I believe experienced radiation poisoning)
Radium watches, tritium gunsights, bananas, carbon 14, …
It’s interesting to me that bananas have persisted as an example of radioactive food. They aren’t actually very radioactive, even by food standards, unless you are getting your bananas from somewhere near the Bikini Atol or something.
Besides the potassium that makes bananas (and other foods with higher concentrations of special K) radioactive, there’s also a certain amount of radioactive iron with a long enough half life for it to be a part of our lives as well as a whole host of other fairly common and fairly stable sources. And, probably most important for all of us, there’s the giant fusion reactor which has us trapped in its gravitational well, which has an output such that it is dangerous to life without shielding even at one AU.
The skins contain a higher amount of potassium than the flesh of the fruit. The reason its reputation persist is it is measurable and accessible.
That and it’s considerably tastier than polonium, uranium, americium, etc. (a joke, of course)
Brazil nuts are more radioactive than bananas I am told
“radioactive materials are banned” – I’m curious where, exactly, the local dirt is banned. Or where there is soil that is not radioactive.
The specific source used for THIS project is regulated (not “banned”) in the US and probably many places.
Worth noting that the claimed 0.4s/day loss is the natural decay rate of the sample, not the actual accuracy of the clock. I really hope [mihai] keeps this running for a while so we can see how well the clock part works. I’ve been running a similar project for more than a year, and I’ve found it pretty difficult to get better than a couple hundred PPM, although I’m using a regular geiger tube.
https://hackaday.io/project/203616-atomic-clock
I think it would be fun to make this as a closed loop system where the radiation source to detector distance is varied with a servo to approximate 1 count per second. This would of course rely on ANOTHER system to provide an accurate 1cps source, but still it would be funny.
Not necessarily.
You could probably do some mechanical math to correct for the (well known) rate of nuclear decay.
nice idea
This won’t work as described on hack a day. CsI(Tl) crystals don’t scintillate off alpha consistently( rarely). Also the alpha particle won’t penetrate the plastic cover of the scintillation crystals.
/Sr Radcon tech who has worked alpha exclusive facility.
Hi! Actually that was the fun part of the challenge when I made this clock. The alpha source is enclosed in the same cover with one of the scintillators, so there’s just a few mm of air between the source and bare scintillator face. I did not mention it in the post but I used GAGG(Ce) for the alpha scintillator, and there is enough scintillation consistency to get a very broad peak from the alphas.
Thank you for the clarification. That will indeed work. I figured some critical information was missing. I’m at work so I all I had time to explore was the hackaday article on this and your previous detector.
Oh, I want a clock like that. I think a random number generator and microcontroller would be easier though.
I think there is a missed opportunity to label the clock with “ATOMIC TIME” , it would really improve it in my opinion.